1. Field of the Invention
The present invention relates to a steering control apparatus for turning steered wheels in accordance with rotation of a steering wheel. More particularly, the invention concerns a steering control apparatus wherein a steering shaft connected to the steering wheel is mechanically separated from a turning mechanism for turning the steered wheels and wherein the turning mechanism is controlled by an electric control device.
2. Related Background Art
An example of such steering control apparatus is described in Japanese Laid-open Patent Application No. Hei 2-85059 and the general configuration thereof is shown in FIG. 4. Steering angle sensor 103 provided on steering shaft 101 detects a steering angle of steering wheel 102 and reaction force mechanism 104, a main element of which is a motor, exerts steering reaction force on the steering shaft 101. Rotation of motor 105 for turning the tires is transmitted through reducing unit 106 to pinion 107 to axially displace rack shaft 108 in mesh with the pinion 107, thereby turning the tires 109. A displacement amount of the rack shaft 108 is detected by turning angle sensor 110 and is supplied, together with a detection signal of the steering angle sensor 103, to controller 100.
The controller 100 of the steering control apparatus constructed in the above structure performs such feedback control of motor 105 that a real steering angle obtained from the detection result of turning angle sensor 110 can be coincident with a target steering angle obtained based on the steering angle detected.
Gains in this feedback control are so set as to be always constant, regardless of the running status of vehicle. This sometimes resulted in failing to perform proper steering control, depending upon the running status of vehicle. For example, if a small gain was set despite circumstances requiring a larger gain (e.g., during running at low speed), the response of control would be slow; conversely, if a large gain was set despite circumstances requiring a smaller gain (e.g., during running at high speed), the sensitivity could be so high as to degrade stability.